Die casting methods are old in the art. The methods permit continuous manufacturing of die cast products with a high degree of quality, such that the methods are commonly used.
In conventional metallic die casting, molten metal is introduced into a shot sleeve or shot chamber. Generally, the molten metal is superheated before it enters the shot sleeve, and thus is introduced to the shot sleeve at a temperature between about 1100.degree. F. and 1600.degree. F., for aluminum, for example. A plunger then slides into the shot sleeve and forces the molten metal into a die cavity. Increased pressure is required to be exerted by the plunger at the end of the fill cycle to compress and force the molten metal in the casting dies. The overall strength of the piece being die cast is dependent, in part, upon the amount of pressure applied by the plunger and upon the initial temperature of the molten metal and its quality. Frequently, tight tolerances are necessary between the plunger and the shot sleeve to minimize any metal blow by around the plunger tip.
Tight tolerances also have the effect of creating additional friction between the plunger and the shot sleeve walls. Further, mechanical and thermal stresses may add additional friction between the plunger and the shot sleeve wall. It is conventional, in cold chamber die casting, that the inside walls of the shot sleeve are lubricated with a lubricant to counteract the frictional forces. It is a goal of the applied lubrication to minimize the wear of the plunger and shot sleeve walls, to prevent blow by and to permit the die casting process to operate continuously.
Conventional lubricants include both solid and liquid materials of various compositions. The liquids may be aqueous based or oil based and may contain various organic and inorganic lubricants. Solid lubricants may include both organic and inorganic materials. The organic materials include a variety of oils, greases and waxes of both natural and synthetic origin. The inorganic materials may include a variety of high pressure lubricants. For example, the inorganic materials may include talc, various nitrides, such as boron nitride, sulfur compounds, such as molybdenum disulfide, silica compounds and may also include graphite and carbon. The inorganic lubricants in particular are inexpensive and highly effective lubricants, as noted by U.S. Pat. No. 5,014,765. However, these materials are typically commercially available in a finely divided particulate form. This finely divided particulate form presents difficulties in handling and dispensing, requiring special methods of application, and may create airborne dust.
Prior U.S. Pat. No. 5,154,839 attempted to solve the problems created with the use of inorganic solid lubricants. The patent discloses the use of an inorganic granulated lubricant which has been coated with an organic polymer or metal soap. While coating the inorganic lubricant with a polymer or metal soap may reduce the dusting problems experienced with the use of prior solid lubricants, the lubricant disclosed by the patent produces a lubricant which may not maintain the integrity of the particles sufficiently and may not be as desirable for use with metering and dispensing apparatus without caking or blocking.
Organic lubricants, including those containing some inorganic material, have an additional problem. In use, these materials frequently generate an open flame and smoke. Organic materials, such as oils and waxes in conventional lubricants, are volatile and flash under the temperature conditions to which these materials are exposed. Frequently a plume of flame and smoke flashes back through the shot hole when the molten metal comes in contact with the applied lubricant.
Applicant is aware of the following U.S. patents, the disclosures of which are incorporated by reference herein.
3,645,319 5,076,339 3,779,305 5,154,839 5,014,765 5,400,921